End cap nucleation of carbon nanotubes

An easily applied graphical approach for facilitating precise tailoring during computational construction of model uncapped or capped single-wall nanotubes or fullerenes is delineated and utilized in this paper. The main enabling concepts are the commonly suggested growth mechanisms for single- and multi-walled carbon nanotubes mediated by end cap structures. The construction protocol described herein can be used to rapidly create any type of armchair, zigzag, chiral or nonchiral defect-free nanotube. Any desired, feasible combination of length and diameter, along with specific placement of hexagonal and pentagonal rings or end caps, can also be controlled. A classification system for end caps is also developed in order to help simplify choosing or describing the particular carbon system under investigation. The suggested methodology is used to systematically model heats of formation of a variety of carbon nanotubes and related fullerenes using AM1 semiempirical calculations. The main factors affecting the calculated physical properties, other than size, are the structures of the various base and terminating end caps. Finally, we comment on the possible relationship of the construction methodology to mechanisms for carbon nanotube nucleation.     

Epitaxial nucleation model for chiral-selective growth of single-walled carbon nanotubes on bimetallic catalyst surfaces

An epitaxial nucleation model for single-walled carbon nanotube (SWCNT) growth on bimetallic catalysts surfaces is reported in support of experimental observations of chiral enrichment. We model the bimetallic catalyst surfaces as a 2D (1 1 1) surface consisting of Ni or a combination of Ni and Fe atoms, with varying average bond length between nearest neighbor atoms which corresponds to the crystal structure of the alloys. The energies associated with nanotube cap formation on these various surfaces are calculated using density functional theory (DFT). We find that certain cap chiralities, such as (8, 4), are more stably bound to a surface that resembles a Ni_0.27Fe_0.73 bimetallic catalyst, whereas other chiralities, such as (9, 4), are more stable on a pure Ni surface. These results help explain the predominance of certain chiralities on specific bimetallic catalysts and provide a potential route to controlling the chirality of as-grown SWCNTs.     

Methanol-mediated growth of carbon nanotubes

A new approach for the growth of carbon nanotubes (CNTs) is reported. In particular, the growth is mediated by adding methanol into n-hexane with dissolved ferrocene. The first mediating effect is that the thermal decomposition of n-hexane and consequent widespread formation of carbon particles can be suppressed. Thus, truly continuous production of CNTs can be realized at high temperature. The second mediating effect is that with increasing methanol addition the proportion of carbon which can be used for CNT growth is decreased. Therefore, CNTs with different diameters or different numbers of graphitic layers can be produced individually.     

Sandwich growth of carbon nanotubes

Direct formation of structures that comprise freestanding CNTs connected to two surfaces was, thus far, not possible. In this article we report a novel approach to grow structured, highly oriented carbon nanotubes that are vertically aligned between a substrate and a massive cover. Growth is feasible at pre-determined, e.g., lithographically defined sites on metallic, semiconducting, or glass substrates. A novel, sandwiched catalyst structure and microwave plasma chemical vapor deposition (CVD) led to the formation of freestanding, small diameter carbon nanotubes. Our new technology offers a simple and scalable pathway to create 3D structured nanotube-based two-terminal electronic devices, device arrays, sensors and corresponding electronic circuits.     

Mechanism growth of multiwalled carbon nanotubes on carbon black

The growth of multiwalled carbon nanotubes (MWCNTs) on carbon black has been studied using a combustion oxygen/acetylene flame method. Different types of carbon black and reaction temperatures were evaluated for the growth of carbon nanotubes. The reaction was stopped after different short duration times of deposition in an attempt to observe the growth of carbon nanotubes. The samples were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and by Raman Spectroscopy. We have observed the transformation of the carbon black surface into graphitic sheets and the start formation of tubes from these graphitic sheets. The length of the tubes is increased but the diameter is decreased with increased deposition times. Carbon nanotubes with 10–20 nm of diameter and a length of about 50 µm are obtained after 1 min deposition. The growth of multiwalled carbon nanotubes on carbon black is a phenomenon that can not be fully explained by carbon nanotubes growth models currently known. Our results lead us to propose a mechanism for the solid-state transformation of the carbon black particles surface into nanotubes in the oxygen/acetylene flame.     

A nucleation and growth study of gold nanowires and nanotubes in polymeric membranes

Through the use of a technique known as template synthesis, it has become possible to synthesize a variety of different materials in the form of nanowires or nanotubes. Dependent upon which type of template is used, either randomly or regularly dispersed nanowires or nanotubes with a wide variety of nanopore diameters and lengths can be created. In this experiment, gold nanowires and nanotubes have been synthesized with diameters of 30, 100, and 800 nm in polycarbonate membranes. The kinetics and characteristics of growth can be greatly altered, dependent upon what operational parameters are employed during deposition. This study looks at the different growth factors that need to be considered when employing the template synthesis approach. These factors include the final expected geometry, the distribution of nucleation sites, the grain size distribution, and the deposition rate.     

Solid-liquid-solid growth mechanism of single-wall carbon nanotubes

Reasons are presented which suggest that the liquefaction of the catalytic particles is a decisive condition for formation of single wall carbon nanotubes (SWNTs) by physical synthesis techniques. It is argued that the SWNT growth mechanism is a kind of solid-liquid-solid graphitization of amorphous carbon or other imperfect carbon forms catalyzed by molten supersaturated carbon-metal nanoparticles. The assumption of low temperature melting of these nanoparticles in contact with amorphous carbon followed by its precipitation in the form of SWNTs allows to explain qualitatively the experimentally observed SWNT growth rates and temperature dependence of the SWNT yield. Guidelines for increasing SWNT yield are proposed.     

MgO-catalyzed growth of N-doped wrinkled carbon nanotubes

We report a MgO-catalyzed growth of N-doped carbon nanotubes (N-CNTs) that are constructed by graphene layers with wrinkles. Introducing NH₃ to a chemical vapor deposition process and keeping a high reaction temperature (∼900 °C) are key factors to the N-CNT growth from MgO. Without N-doping or at a lower temperature, only graphene sheets deposited on MgO surfaces were obtained. Compared to the Fe-catalyzed N-CNTs, the MgO-catalyzed N-CNTs have larger diameters, thinner walls, more surface wrinkles and open ends with polygonal graphene edges. Because carbon dissolves into MgO in a very limited amount, the structural defects and broken sites formed in the graphene layers due to N-doping might have contributed to the formation of new graphene layers and thus lead to the growth of the wrinkled tubes.     

碳纳米管对重金属的吸附技术进展研究

综述了一种涉及碳纳米管(CNT)被用作吸附剂去除水中重金属的特定技术。吸附法被认为是从水中去除重金属离子的一种合格技术,使用深共熔溶剂(DES)作为功能化剂以改善CNT,从而提高了CNT的吸附性能,能去除水中重金属。相对于离子液体(IL),DES具有许多优势,包括成本低廉,材料容易获得以及对环境友好,这表明使用带有DES功能化的CNT作为吸附剂能够带来最有效的水中重金属吸附效果。     

Solid state growth mechanisms for carbon nanotubes

The mechanisms by which carbon nanotubes nucleate and grow remain poorly understood. This paper reviews the models which have been proposed to explain nanotube growth in the arc-evaporation and laser-vaporisation processes. Many of the early models assumed that growth is a gas phase phenomenon but there is growing experimental evidence that the formation of both multiwalled and single-walled tubes involves a solid state transformation. Heating rate also seems to be important in promoting nanotube growth.     

Gadolinium and europium catalyzed growth of single-walled carbon nanotubes

The inner transition metals, gadolinium (Gd) and europium (Eu) have been shown to catalyze the growth of single-walled carbon nanotubes (SWCNTs) using chemical vapor deposition. The Gd and Eu nanocatalysts, prepared using a diblock copolymer templating method and characterized by atomic force microscopy, were uniformly spaced over a large deposition area with an average diameter of 1.9 nm and narrow size distribution. Characterization by transmission electron microscopy and Raman spectroscopy confirms the presence of SWCNTs catalyzed by Gd and Eu with an average diameter of 2.05 nm.     

Novel growth of carbon nanotubes on nickel nanowires

A novel growth phenomenon is presented in this paper where carbon nanotubes (CNT) were grown successfully on nickel (Ni) nanowire using chemical vapour deposition technique. The decomposed carbon from ethylene diffused through the surface of nanowires and precipitated into hollow cylindrical carbon structures. Nanotubes of various lengths are found to have grown along the length of the outer side of the nanowires and were firmly rooted to their walls. The presence of a thin layer of oxide (~ 3 nm) on the top surface of nanowires is believed to have promoted the growth of CNT. Raman, X-ray photoelectron (XPS) and electron energy loss spectroscopy (EELS) were conducted in order to understand the formation of nanotubes and verify their presence, their level of crystallinity and chemical bonding structure with nanowires. This hybrid nanostructure is also found to have ferromagnetic behaviour, which can be applied in devices such as magnetic sensors and spintronic devices that combine the unique properties of CNT and Ni nanowires.     

Low temperature, non-isothermal growth of carbon nanotubes

Non-isothermal growth of carbon nanotubes (CNTs) was studied in order to obtain the activation energy. CNTs were grown on an Fe-Si catalyst using microwave plasma-enhanced chemical vapor deposition of CH₄. During the growth, the substrate was heated by the plasma such that its temperature increased with the growth until equilibrium was reached. In other words, the CNT growth took place under simultaneously increased time and temperature. This has resulted in different growth kinetics from previously published studies. We have therefore examined the growth kinetics using an empirical function for CNTs grown on three different substrates and derived the relevant activation energies based on the empirical function.     

Combined growth of carbon nanotubes and carbon nanowalls by plasma-enhanced chemical vapor deposition

A technique is reported for the combined growth of carbon nanotubes (CNTs) and carbon nanowalls (CNWs) by plasma-enhanced chemical vapor deposition. The observation serves as a direct proof of a close correlation between the growth of both materials because both are obtained in a single experiment without making any changes to the growth parameters. The growth of freestanding CNTs is driven by a nickel catalyst deposited on an oxidized silicon wafer. It is assumed that the remaining carbon radicals are inserted in the sidewalls and tips of the tubes after the saturation of the catalyst by abundant carbon, thereby forming a CNW layer on top of the CNTs. A possible growth scheme, based on qualitative analysis by electron microscopy, Raman spectroscopy and X-ray diffraction, is presented. It is further shown that the CNWs easily detach by dipping the sample into water, while the CNTs remain attached to the sample.     

The importance of catalyst oxidation for the growth of carbon nanotubes on Si substrates

Iron catalyst in the form of a thin layer was deposited directly onto silicon wafers in order to grow carbon nanotubes by a chemical vapor deposition process. The chemical and morphological transformations undergone during the process by the iron were monitored in situ by photoelectron spectroscopy and ex situ by SEM. We found that the growth of carbon nanotubes was successful only when the iron catalyst was previously oxidized. Metallic iron transformed to embedded iron silicide structures, unable to initiate nanotube growth. On the other hand, pre-oxidized iron led to the formation of superficial particles and showed only a partial conversion to silicide, which resulted in carbon nanotube growth, even at very low pressures.